Need help with nRF52 I2S EasyDMA - RXD Buffer Pointer

Question

Hi, 
 I'm trying to setup NRF52 with a microphone (ICS-43432) using I2S and easyDMA. I'm new to embedded software development, and would very much appreciate your help and patience. 
 My plan is to use just one (left) channel for receiving the data from the microphone and transmit it over BLE using the S132 stack. 
 From the code snippet in the I2S section of the product specification, I gather that the RX data pointer for easyDMA needs to be configured as follows: 
 NRF_I2S->RXD.PTR = my_rx_buf;

NRF_I2S->RXTXD.MAXCNT = MY_BUF_SIZE;
 
 Three questions: 
 
 
 What should my_rx_buf look like? Does it have to be statically allocated? Does it have to be an ArrayList with a buffer like in the "SPIM with EasyDMA" section?
A code snippet for my_rx_buf would be very helpful. 
 
 
 What should be the buffer size if I've configured my code for a sample rate of 25 ks/s (MCKFREQ=3.2 MHz and Ratio=128)? 
 
 
 Is I2S fully functional in PCA10040 (nRF52 kit received from the nRF52 global tour)? 
 
 
 Thank you again for your excellent guidance & help! 
 Regards,
Shashi
Austin TX 
 P.S.: You're may be finishing up the samples for I2S in the nRF52 SDK. Is there a way to get the preview-version of those examples?

Anders Strand · Accepted Answer

Hello. 
 1 -
This should be a statically allocated area of RAM. It will contain data in "chunks" of 32 bits. Whether you implement an ArrayList structure type with 4 bytes, or one array of uint32_t is up to you. If you choose the latter, you must extract the data manually from each 32 bit chunk. The allocation will look like this: 
 static uint32_t m_rx_buf[I2S_BUFFER_SIZE];
 
 Remember that you can choose whether the data is going to be 8,16 or 24 bits. If you choose 16 bits, you can extract the two 16 bit words from a uint32_t varible "buffer" like this: 
 uint16_t actual_sample_left = ((uint16_t const *)buffer)[0];
 uint16_t actual_sample_right = ((uint16_t const *)buffer)[1];
 
 2 - This really depends on how you plan to do the sampling. In BLE, you can send 20 bytes in one packet. It will make sense to gather 20 bytes at a time, and then send these bytes out. However, if you want to continue to sample audio while reading the previous sample from RAM, you must implement double buffering. You will in this case need two buffer of 20 bytes each, and switch between the two buffers when you get an interrupt saying that 20 bytes have been received. 
 3 - Yes. You can see it on the list of fixed anomalies in the errata . 
 For others, here is the link to the i2s section of the OPS 
 The i2s driver and example project will be released in the next SDK. (Probably just before Christmas this year) 
 adpcm codec: dvi_adpcm.rar

Need help with nRF52 I2S EasyDMA - RXD Buffer Pointer

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